It is known that such an installation includes ducts or conduits through which hot air and cold air passes and in which the flow of air is adjusted by flaps rotatably mounted in the ducts to vary the cross section of the air passage through the duct or conduit. These flaps are generally actuated from the vehicle dash board by means of control cables that slide in protective sheaths.
It is important for the flaps to be sufficiently rigid against bending and twisting to avoid being bent or twisted about their axis of rotation. Up to the present, they have therefore been made either of relatively thick (at least 1 mm thick) galvanised metal, or of rigid plastics material, generally including fibre reinforcement and provided with stiffening ribs.
In both cases the flaps are heavy and can only be rotated by applying a relatively large force to their control cables.
Further, flap rotation control must be substantially irreversible, ie. the flow of air through the corresponding duct must not change the position of the flaps. To ensure this, brake means between the cable and its sheath are provided to apply a sufficiently large resistance to movement to ensure that a flap cannot be moved solely by the action of the air flow in the duct. This braking force adds to the force needed to overcome the inertia due to the weight of the flap so that the flap controlling force needed from the vehicle dash board is large and the controls appear to be stiff.
Preferred embodiments o the present invention avoid this drawback, providing a control flap which is very light in weight but which is nonetheless sufficiently rigid.